The present invention relates generally to a mechano-electrical converter. In particular, the invention concerns a mechano-electrical converter for converting mechanical loads into electrical signals by using a semiconductor strain gauge.
There have been proposed various mechano-electrical converters for different applications. A load sensor which incorporates a cantilever having a semiconductor strain gauge bonded thereto is used in electronic weghing machines and the like. Further, various pressure sensors are known in which pressure-sensitive semiconductor diaphragms are made use of. A typical one of the pressure-sensitive semiconductor diaphragms is composed of a semiconductor wafer having a central thin portion at which piezo-resistors are integrally formed by doping an impurity of opposite conductivity type to that of the wafer. When the diaphragm is deformed under influence of external pressure applied thereto, the resistance of the piezo-resistor undergoes a corresponding variation, providing a measure of the pressure as applied. With a structure in which the thin portion of the pressure-sensitive diaphragm is covered with a fluidal or deformable material such as silicone oil or silicone rubber, it is possible to measure a load in terms of a variation in pressure. The thin portion or region of the semiconductor diaphragm may be of rectangular, circular or any other suitable configuration and is usually formed with at least two piezo-resistors which are electrically connected in a form of bridge circuit or inserted in arms of the bridge circuit.
In U.S. Pat. No. 4,135,408 issued to Giovanni, there is proposed a silicon diaphragm which is constituted by a silicon wafer having an annular groove defining a thin annular area which serves as a pressure sensitive resilient member. A back plate having a slightly recessed center portion is provided in combination with the silicon diaphragm, wherein a central boss of the diaphragm functions as a limit stop.
In U.S. Pat. No. 4,364,276 assigned to the present assignee, a differential pressure measuring transducer is taught in which a semiconductor diaphragm also having a thin annular area is used, wherein the diaphragm having a central boss area formed thinner than the peripheral support flange is mounted on a flat metal support through an interposed flat glass support.
Further, U.S. Pat. No. 3,456,226 discloses a mechano-electrical transducer of a structure in which a central thin portion of a silicon diaphragm is adapted to be pressed by a post bar. However, this structure which would possibly be destroyed under excessive pressure is not suited for use as the load cell.
On the other hand, in the case of the load cell of the type in which a load to be measured is converted into a pressure which is then actually measured, a conversion error is likely to be involved in the result of the measurement.
Accordingly, an object of the present invention is to provide a semiconductor load converter which assures an improved accuracy of measurement and can enjoy a long use life.